Semantic externalism is the view that the meaning of a term, and the understanding of a language in general, is relational; determined by factors external to the speaker and bound up with our environment.The debate about semantic externalism is one of the most important but difficult topics in philosophy of mind and language, and has consequences for philosophical and empirical theories about the mind, and the role of social institutions and the physical environment in constituting language. In this long-needed book, Jesper Kallestrup provides an invaluable map of the problem. Beginning with a thorough introduction to the theories of descriptivism and referentialism and the work of Frege and Kripke, Kallestrup moves on to analyse Putnam's 'Twin Earth' argument. He also discusses how semantic externalism is at the heart of important topics such as narrow and wide content, self-knowledge, and mental causation.Including chapter summaries, a glossary of terms, and an annotated guide to further reading, Semantic Externalism an ideal guide for students studying philosophy of language and philosophy of mind.